It has been proposed to upgrade heavy hydrocarbons via hydroprocessing in which the hydrocarbons are admixed with an active catalyst composition in liquefied slurry form. As disclosed in U.S. Published Application No. 2007/0140927, the disclosure of which is incorporated herein by reference, a feed of heavy hydrocarbons and catalyst slurry is introduced into the lower portion of a reactor chamber, along with hydrogen in a gas phase. Those components travel upwardly within the chamber, enabling the hydrogen to react with, and hydrogenate, the hydrocarbons. Near an upper portion of the chamber, the hydrogenated hydrocarbons are removed as is excess hydrogen gas.
A flow of liquefied catalyst slurry and residual hydrogen gas is recirculated within the chamber through a vertically oriented downcomer in the chamber under the action of a recirculation pump. The downcomer has traditionally been located coaxially in the center of the reaction chamber. The mixing which occurs in the downcomer tends to keep the catalyst concentration profile and the temperature profile generally uniform along the height of the reactor.
Multi-phase mixture exiting the downcomer is recirculated, e.g., back into the reaction chamber or elsewhere. The recirculation is effected by a pump, the efficiency of which is reduced by the presence of gas bubbles in the mixture. Therefore, at or near its upper end the downcomer is typically provided with a degassing section, e.g., in the form of a generally frusto-conical pan which is upwardly open and leads downwardly to a usually cylindrical transport section of the downcomer. Due to the degassing section being of relatively large diameter, travel of the multi-phase mixture therein is slower than the natural ascension velocity of the hydrogen gas bubbles, thereby facilitating escape of the bubbles from the rest of the multi-phase flow.
Notwithstanding those efforts, gas bubbles will still exit the downcomer and reduce the pump efficiency, or possibly even damage the pump. It would, therefore, be desirable to further separate gas bubbles from the mixture being recirculated.
It would further be desirable to reduce the power requirements of the pump.